Not applicable.
The present invention relates generally to processing of semiconductor devices, and specifically to methods and apparatus for removal of foreign particles from semiconductor wafers and masks.
Removal of contaminants from semiconductor wafers is a matter of great concern in integrated circuit manufacturing. As the critical dimensions of circuit features become ever smaller, the presence of even a minute foreign particle on the wafer during processing can cause a fatal defect in the circuit. Similar concerns affect other elements used in the manufacturing process, such as masks and reticles.
Various methods are known in the art for stripping and cleaning foreign matter from the surfaces of wafers and masks, while avoiding damage to the surface itself. For example, U.S. Pat. No. 4,980,536, whose disclosure is incorporated herein by reference, describes a method and apparatus for removal of particles from solid-state surfaces by laser bombardment. U.S. Pat. Nos. 5,099,557 and 5,024,968, whose disclosures are also incorporated herein by reference, describe a method and apparatus for removing surface contaminants from a substrate by high-energy irradiation. The substrate is irradiated by a laser with sufficient energy to release the particles while an inert gas flows across the wafer surface to carry away the released particles.
U.S. Pat. No. 4,987,286, whose disclosure is likewise incorporated herein by reference, describes a method and apparatus for removing minute particles (as small as submicron) from a surface to which they are adhered. An energy transfer medium, typically a fluid, is interposed between each particle to be removed and the surface. The medium is irradiated with laser energy and absorbs sufficient energy to cause explosive evaporation, thereby dislodging the particles.
One particularly bothersome type of contamination that is found on semiconductor wafers is residues of photoresist left over from a preceding photolithography step. U.S. Pat. No. 5,114,834, whose disclosure is incorporated herein by reference, describes a process and system for stripping this photoresist using a high-intensity pulsed laser. The laser beam is swept over the entire wafer surface so as to ablate the photoresist. The laser process can also be effected in a reactive atmosphere, using gases such as oxygen, ozone, oxygen compounds, nitrogen trifluoride (NF3), etc., to aid in the decomposition and removal of the photoresist.
Various methods are known in the art for localizing defects on patterned wafers. A summary of these methods is present in an article entitled xe2x80x9cDefect Detection on Patterned Wafers,xe2x80x9d in Semiconductor International (May, 1997), pp. 64-70, which is incorporated herein by reference. There are many patents that describe methods and apparatus for defect localization, for example, U.S. Pat. Nos. 5,264,912 and 4,628,531, whose disclosures are incorporated herein by reference. Foreign particles are one type of defects that can be detected using these methods.
U.S. Pat. No. 5,023,424, whose disclosure is incorporated herein by reference, describes a method and apparatus using laser-induced shock waves to dislodge particles from a wafer surface. A particle detector is used to locate the positions of particles on the wafer surface. A laser beam is then focused at a point above the wafer surface near the position of each of the particles, in order to produce gas-borne shock waves with peak pressure gradients sufficient to dislodge and remove the particles. It is noted that the particles must be dislodged by the shock wave, rather than vaporized due to absorption of the laser radiation. U.S. Pat. No. 5,023,424 further notes that immersion of the surface in a liquid (as in the above-mentioned U.S. Pat. No. 4,987,286, for example) is unsuitable for use in removing small numbers of microscopic particles.
It is an object of the present invention to provide methods and apparatus for efficient removal of contaminants from solid-state surfaces, and particularly for removal of microscopic particles from semiconductor wafers and other elements used in semiconductor device production. The wafers may be bare, or they may have layers formed on their surface, whether patterned or unpatterned.
It is a further object of some aspects of the present invention to provide improved methods and apparatus for targeted removal of contaminant particles from a surface based on prior localization of the particles.
It is yet a further object of some aspects of the present invention to provide apparatus and methods for removal of particles from a surface with reduced probability that a particle, once removed, will settle back onto another area of the surface.
In preferred embodiments of the present invention, a substrate whose surface is to be cleaned, such as a semiconductor wafer, is mounted on a rotating chuck. A laser beam is scanned in a radial direction, between the center of the substrate and its periphery, thus defining a narrow, radial zone of irradiation. Any point on the substrate can be positioned in the zone simply by rotating the chuck. In this manner, the laser beam can reach substantially any or every point on the substrate surface by means of a simple one-dimensional scan, in cooperation with the chuck rotation. Preferably, gases having reactive properties and/or inert gases are flowed through the zone of irradiation in order to assist in ablating contaminants and in carrying particles away from the surface. The direction and characteristics of the gas flow are arranged so that the particles are carried away via the shortest possible path, thereby reducing the probability of their settling back onto the surface. The small dimensions of the irradiated zone are significant in shortening the particle removal path, as well as in reducing the quantity of gas necessary to achieve the desired effects.
In some preferred embodiments of the present invention, a particle localization system is used to determine coordinates of the particles that must be removed from the surface. These coordinates are preferably converted to polar coordinates, for use in driving the rotation of the chuck and scanning of the laser beam. The laser beam is directed to irradiate the surface at the locations indicated by the coordinates. Absorption of the laser beam energy at the surface, whether by the particles, the substrate or a material on the substrate, imparts sufficient energy to drive the particles off the surface substantially without damage to the surface itself. Optionally, a fluid or other energy transfer medium is applied to the surface, as described, for example, in the above-mentioned U.S. Pat. No. 4,987,286, in order to promote removal of the particles by local explosive evaporation.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for removing particles from the surface of a substrate, including:
determining respective position coordinates of the particles on the surface; and
directing a beam of electromagnetic energy at the coordinates of each of the particles in turn, such that absorption of the electromagnetic energy at the surface causes the particles to be dislodged from the surface substantially without damage to the surface itself.
Preferably, determining the position coordinates includes determining polar coordinates of the particles relative to a reference feature of the substrate. Most preferably, directing the beam of electromagnetic energy includes rotating the substrate about an origin of the coordinates and scanning the beam in a radial direction with respect to the origin, responsive to the polar coordinates. Typically, determining the polar coordinates includes converting Cartesian coordinates of the particles determined by a particle detection system to the polar coordinates.
In a preferred embodiment, absorption of the electromagnetic energy causes ablation of the particles from the surface.
In another preferred embodiment, the method includes interposing an energy transfer medium between the particles to be removed and the surface, wherein absorption of the beam of electromagnetic energy causes local evaporation of the medium, thereby dislodging the particles.
In yet another, preferred embodiment, the method includes flowing a gas, preferably a gas having reactive properties, in the vicinity of the surface, which gas cooperates with the absorbed energy in removing the dislodged particles from the surface.
Preferably, directing the beam of electromagnetic energy includes directing a laser beam at the surface, and the substrate includes a semiconductor wafer.
There is further provided, in accordance with a preferred embodiment of the present invention, apparatus for detecting and removing particles from the surface of a substrate, including:
a particle localization device, adapted to determine respective position coordinates of the particles on the surface; and
a particle removal device, coupled to receive the position coordinates from the particle localization device, and including an energy source which is configured to direct a beam of electromagnetic energy at the coordinates of each of the particles in turn, such that absorption of the electromagnetic energy at the surface causes the particles to be dislodged from the surface substantially without damage to the surface itself.
There is also provided, in accordance with a preferred embodiment of the present invention, apparatus for removing particles from the surface of a substrate, including:
a coordinate processor, adapted to receive input position coordinates of the particles on the surface and to determine, responsive to the input coordinates, output position coordinates for controlling the apparatus; and
an energy source, which is configured to direct a beam of electromagnetic energy at each of the particles in turn, responsive to the output position coordinates, such that absorption of the energy at the surface causes the particles to be dislodged from the surface substantially without damage to the surface itself.
Preferably, the output position coordinates include polar coordinates of the particles relative to a reference feature of the substrate. Further preferably, the apparatus includes a rotating chuck, which is configured to receive the substrate and to rotate the substrate about an origin of the polar coordinates so as to position each of the particles in turn under the beam of electromagnetic energy. Most preferably, the beam is scanned in a radial direction with respect to the origin, responsive to the polar coordinates.
There is moreover provided, in accordance with a preferred embodiment of the present invention, apparatus for removing contaminants from the surface of a substrate, including:
a rotating chuck, which is configured to receive the substrate and to rotate the substrate about an axis of the chuck; and
an energy source which is configured to direct a beam of electromagnetic energy over the surface of the substrate, causing the contaminants to be dislodged from the surface, and to scan the beam in a radial direction with respect to the axis, in cooperation with the rotation of the chuck, so as to impinge upon substantially any point on the surface from which the contaminants are to be removed.
Preferably, the apparatus includes a chamber, which is configured to receive the substrate and to form an encapsulated region over a portion of the surface at which the beam of energy is directed. Further preferably, the chamber includes a window, forming one side of the encapsulated region, through which window the beam of energy passes to impinge upon the surface. Most preferably, the encapsulated region contains only a portion of the surface of the substrate.
Preferably, the chamber includes one or more gas ports communicating with the encapsulated region so as to flow a gas therethrough, wherein the gas cooperates with the absorbed energy in removing the dislodged particles from the surface. Further preferably, the one or more gas ports include an inlet port for feeding a gas having reactive properties into the encapsulated region and an outlet port for removing exhaust gas from the encapsulated region. Alternatively or additionally, the one or more gas ports include inlet and outlet ports through which an inert gas is introduced into the chamber so as to generate a laminar flow of the gas over the surface of the substrate. Preferably, the gas flow through the encapsulated region carries away the contaminants dislodged from the surface, and wherein the gas ports are configured so that the gas flows in such a direction as to generally minimize a distance over which the contaminants must be carried before being removed from a vicinity of the surface.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a method for removing contaminants from the surface of a substrate, including:
rotating the substrate about an axis thereof;
directing a beam of electromagnetic energy over the surface of the substrate, causing the contaminants to be dislodged from the surface; and
scanning the beam in a radial direction with respect to the axis, in cooperation with rotating the substrate, so that the beam impinges upon substantially any point on the surface from which the contaminants are to be removed.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: